What about this?

What about this? What does "which" refer to here? "commonsense knowledge" or "weakness"?

"Although commonsense knowledge may have merit, it also has weaknesses, not the least of which is that it often contradicts itself. For example, we hear that people who are similar will like one another (“Birds of a feather flock together”) but also that persons who are dissimilar will like each other (“Opposites attract”)."